The present disclosure is directed to systems for generating audible sound, such as speaker-based systems, and methods of using such systems. In particular, the present disclosure is directed to systems for generating audible sound waves from ultrasonic waves using parametric interactions.
Conventional parametric speakers produce modulated ultrasonic waves, which in turn demodulate through a non-linear medium to generate highly-directional audible sound waves. As illustrated in FIG. 1, a conventional parametric speaker, commonly referred to as an audio spotlight, typically includes an array 10 of planar transducers that emit collimated ultrasonic waves 12. This generates pressure wavefronts 12a, which are fairly steady in the collimated path. While passing through a non-linear medium, such as air, the medium gradually demodulates the ultrasonic waves 12 via parametric interaction to produce audible sound waves within a cylindrical conversion column 14.
Using an optics analogy, the collimated ultrasonic waves 12 emitted from transducer array 10 have a low numerical aperture, such as less than 0.05. This limits the lateral area over which the generated audible sound may be heard by a listener. For example, a person standing at location 16, outside of conversion column 14, would not hear the audible sound. However, a person standing at location 18, within conversion column 14, would hear the audible sound.
Parametric conversion efficiency is proportional to the sound pressure level of the air through which ultrasonic waves 12 travel. A sound wave having a peak pressure of two atmospheres and a trough pressure of vacuum will exhibit a sound pressure level of 194 decibels. At this sound pressure level, the parametric conversion efficiency in air to produce audible sound waves from ultrasonic waves 12 is highly efficient. However, most low-cost parametric speakers only operate between about 110 decibels and 140 decibels, and make up for their lack of efficiency with long interaction volumes, such as in cylindrical column 14.
The long interaction volume in cylindrical column 14 limits how transducer array 10 may be effectively used. For example, if transducer array 10 was intended to present audible content about a particular product in a retail store, transducer array 10 would typically be emit ultrasonic waves 12 vertically downward from a ceiling location of the retail store. The audible sound generated from the demodulated ultrasonic waves would then reflect from the floor to a person standing directly below transducer array 10. However, this does not direct the listener's attention to the intended product. Rather, the listener's attention will be directed to transducer array 10.
Alternatively, if transducer array 10 were otherwise positioned next to the intended product, and oriented to emit ultrasonic waves 12 horizontally, cylindrical column 14 may extend across the entire retail store. If heard by a listener across the retail store, the listener may become confused about what product the audible content is referring to, which is undesirable. As such, there is an ongoing need for parametric systems that produce audible sounds with good parametric conversion efficiencies and that also direct a listener's attention to an intended product or point in space.